This invention relates to the preparation of thiol alkoxylates by the addition reaction of alkylene oxides with thiols. More specifically, this invention is directed to an improved process for conducting the alkoxylation reaction of alkane thiols with alkylene oxides in the presence of certain basic catalysts and certain reaction promoters.
Under conventional practice, thiol alkoxylates have been typically prepared by the reaction of alkylene oxides with alkane thiols in the presence of either acid or base catalysts. In the particular case of the preparation of a thiol ethoxylate (represented by formula III below) the addition of a number (n) of ethylene oxide molecules (formula II) to a single thiol molecule (formula I) is illustrated by the equation ##STR1## wherein R is alkyl and n is an integer equal to or greater than one. The product of any such alkoxylation reaction is a mixture of various thiol alkoxylate molecules having a variety of alkylene oxide adducts, i.e., a mixture of compounds with different values of n.
Thiol alkoxylates are known materials having recognized utility as surfactants and as intermediates in the synthesis of other organic compounds. Thiol alkoxylates in which the alkyl group has a number of carbon atoms in the detergent range, i.e., from about 8 to 20, have attracted particular interest, for instance as components of commercial detergent formulations and as spermicidal agents.
The present invention is directed to improvement in the thiol alkoxylate preparation process which utilizes certain basic catalysts, particularly certain alkali and alkaline earth metal catalysts. In one aspect, this improvement specifically relates to enhanced selectivity of the alkoxylation reaction for the desired thiol alkoxylates. In the presence of the basic catalysts, alkylene oxides tend to react to form polyalkylene glycols, for example, polyethylene glycols of the formula HO(CH.sub.2 CH.sub.2 O).sub.y H, wherein y is an integer greater than 1. The formation of these byproducts not only represents a loss of alkylene oxide reactant but also results in an alkoxylate product of lower quality. In another aspect, the invention is directed to improvements relating to the processing and handling of the alkoxylation reaction mixture. Under common conventional practice, the catalyst is mixed with the liquid thiol reactant and this mixture is then contacted with the gaseous alkylene oxide reactant. The alkali and alkaline earth metal catalysts (or the compounds to which they are converted in the presence of thiol) are insoluble in the thiol reactant, even at typically low catalyst concentrations. The heterogeneous mixtures which result tend to interfere with processing, for instance, by plugging transfer lines to the alkoxylation reactor and fouling heat transfer surfaces.